1. Field of the Invention
The present invention relates to a pattern for castings and castings produced by the pattern of the invention, and more particularly to a pattern for castings which is provided with a suitable multistage draft that improves clamping performance and pattern drawing performance at corner portions of the parting plane of the pattern to reduce the chance of mold drop.
2. Description of the Prior Art
FIG. 5 shows a conventional pattern for castings. The pattern is provided with a single draft having an angle 8' of generally 1-3 degrees for a smooth pattern draw. A corner portion 9' at a parting plane of the pattern 1' is also provided with a chamfer R of a suitable size (conventionally 1-3 mm) to avoid improper mold clamping as well as mold drop when drawing the pattern. Mold drop occurs in the case that an unexpected force is transmitted to the corner portion 9, at the parting plane 5' when the pattern is not drawn perpendicularly to the pattern plane.
The provision of the chamfer R considerably improves the mold clamping performance at the corner portion 9' but causes sharp and peaked acute casting fins 6 at the tip end of the casting as shown in FIG. 7. There is a possibility of workers hurting their hands on the acute casting fins 6. Also, the casting fins 6 may be caught by transferring devices as well as jigs and tools in subsequent machining and assembly operations causing various troubles. In order to avoid these problems the casting fins must first be removed before the subsequent processes can take place, which requires a considerable amount of finishing time and cost. Furthermore, chamfers R are absolutely unnecessary from the point of view of the physical dimensions of the final product.
As shown in FIG. 6, some patterns are made with edges having a substantially right angle at the corner portion 9' rather than the chamfer R. However in these cases, the clamping performance is by no means adequate at these corner portions, thereby causing mold drop when drawing the pattern, as well as causing casting fins which must be removed as described above.
As illustrated in FIG. 4, Japanese Utility Model Publication No. 53-50728 discloses a pattern for pipe joints or valves which is provided with a draft in the form of tangent lines 10 intercepting the parting plane 5, drawn from the outer surface of a circular body of the pipe. By this arrangement, the draft in the vicinity of the parting plane 5 is not strictly vertical, which not only avoids the mold drop at the corner portion 9 but also ensures improved mold clamping performance at the corner portion 9. This arrangement may be applied to the castings such as the pipe joints and valves in which the height of the casting is not large, but may not be applied to conventional castings that are vertical (at right angle to the parting plane) and relatively tall in height.